This invention relates to a process and a device for the generation of a dense fog of micrometric and submicrometric droplets.
In particularly, it is applicable to the generation of extreme ultraviolet radiation also called xe2x80x9cEUV radiationxe2x80x9d.
It is radiation with a wave length within the range between 8 and 25 nanometers.
The EUV radiation produced according to this invention has many applications, particularly in science of materials, microscopy and particularly lithography, to make integrated circuits with a very large scale integration (VLSI).
Other applications include the surface deposition of aggregates, for which large and hot aggregates are more suitable than small and cold aggregates generated by all devices according to prior art.
Many techniques are known for the production of EUV radiation, for example techniques consisting of using a laser beam to irradiate a target placed in a vacuum.
When the illumination from the laser beam is sufficiently intense, the target is strongly ionised. A plasma is thus created in which a number of particles, excited and/or ionised by the electromagnetic field resulting from the laser beam and collisions with other plasma particles are de-excited or recombined while emitting light in the extreme ultraviolet. Thus, a sort of frequency conversion takes place in the heart of the target.
Several types of targets are known that can give a high frequency conversion efficiency capable of producing the most intense possible EUV light.
In particular, for the lithography of integrated circuits, it is necessary to find a target, that can be irradiated by a laser for the production of light in the extreme ultraviolet and that is compatible with the use of lithography for industrial applications. This type of target must satisfy the following requirements:
Debris caused by interaction between the target and the laser beam must be minimised in order to avoid damaging the very expensive optics of the lithography apparatus.
It must be possible to supply the material from which the target is made continuously or in pulsed mode at high frequency, for example of the order of 1 kHz. Therefore, this material must be continuously renewable.
Since the laser is focused on the target, the quantity of irradiated material must be sufficiently high to enable intense emission in the extreme ultraviolet. This imposes two conditions on the target. Firstly, the dimensions of this target must not be too small. Secondly, the average density of the target must be sufficiently large.
The target must be placed in a vacuum, in an area in which the pressure is for example of the order of 10xe2x88x922 Pa. The pumping means used to give this pressure must be free of vibrations.
The energy transfer between the laser and the target must be efficient to guarantee a high conversion efficiency.
The target generation device must be reliable in the long term. In particular, the interaction between the laser and the target must take place at a sufficiently large distance from this device to prevent premature wear in the device by the impact of ions ejected from the plasma. This leads to the generation of solid debris originating from erosion of the nozzle.
Generation of EUV radiation by irradiation of a dense jet of xenon aggregates onto which a beam emitted by a nanosecond laser is focused, is divulged in document [1] which, like the other documents mentioned later, is mentioned at the end of this description.
Note that these xenon aggregates are submicrometric grains and they are obtained by condensation of xenon during adiabatic expansion through a nozzle in a vacuum chamber.
Irradiation of these aggregates by a laser beam in the near infrared or the near ultraviolet produces a plasma that emits higher energy light located in the extreme ultraviolet. Coupling between the laser and the target, and therefore the efficiency of this conversion process, are particularly important in the case of irradiation of a jet of xenon aggregates.
A large amount of the laser light is thus absorbed, which encourages creation of a plasma by heating of the aggregates. This efficiency of the conversion process is due to the very small dimensions of the aggregates (smaller than 0.1 xcexcm) which encourages almost complete penetration of laser light into each aggregate.
Furthermore, the local density of atoms in each aggregate is very high, such that a large number of atoms participate. Furthermore, the large number of aggregates comprising a sufficiently high average number of atoms within the laser beam focusing area results in a very strong emission in the extreme ultraviolet.
Another advantage of an EUV radiation source based on irradiation of a jet of aggregates by a nanosecond laser lies in the almost complete lack of material debris, in other words fast material fragments emitted by the irradiated jet, since these debris could damage the EUV radiation collection optics.
However, large quantities of material debris can be produced by erosion of the nozzle when it is placed too close to the illuminated area. Thus, the information given in documents [1] and [7] divulges that the illuminated area should be placed at a small distance from the nozzle (1 to 2 mm), which causes the generation of large quantities of debris due to erosion of the nozzle.
The use of a jet that forms a renewable target makes it possible to work at a high frequency (of the order of 1 kHz or more), which is suitable for lithography apparatus for the manufacture of very large scale integrated (VLSI) circuits.
The use of xenon as the aggregation gas gives the best results for the emission of extreme ultraviolet since xenon is the gas (a) which gives one of the highest degrees of condensation and thus generates a sufficiently high average size for the aggregates and (b) which has a large number of emission rays within the spectral range considered.
Furthermore, the interaction area between the laser beam and the aggregates jet is small, so that a maximum amount of EUV radiation can be condensed while minimizing optical aberrations.
However, the EUV radiation source that is divulged in document [1], has a number of disadvantages:
The expansion nozzle has to be strongly cooled, which requires significant cryogenic means,
The reliability of the aggregates generation device when the nozzle is cooled is reduced by the presence of a large temperature gradient between the cooled end of the nozzle and the movement mechanism of the pulsed valve at which local temperature rises occur.
Operation at high frequency (of the order of 1 kHz) requires high gas flows and consequently very powerful pumping means that could induce vibrations harmful to the alignment of the optics for a lithography apparatus comprising the aggregate generation device.
In the case of xenon, gas recovery means are necessary to minimise costs that could become prohibitive at an industrial scale.
The EUV radiation generation process must only occur within a small area with a diameter smaller than 1 mm. Consequently, all that is actually used is a small quantity of gas inside the jet. However according to document [1], FIG. 5 and document [7], FIG. 5, the density of aggregates decreases sharply when the distance from the nozzle increases. This is why excitation by the laser beam must take place in the immediate vicinity of the nozzle which causes severe erosion of this nozzle (which is usually metallic) by the impact of ions output from the plasma. Erosion of the nozzle significantly reduces its life and therefore the reliability of the EUV radiation source and generates large quantities of debris that could deteriorate the optics and the mask of the lithography apparatus.
Document [2] divulges an EUV radiation source that uses a jet of ice micro-crystals as the target. This consists of a sequence of micro-crystals at a very high repetition frequency in which each micro-crystal typically has a diameter of several tens of micrometers.
These micro-crystals are too large for penetration of the excitation laser beam to be complete, unlike what happens with xenon aggregates. A reduction of the diameter of each micro-crystal would improve penetration, but the efficiency would drop because the number of EUV photon emitters in the plasma would-be reduced.
Therefore, the technique described in document [2] does not satisfy the criteria for obtaining a sufficiently intense EUV radiation source.
Document [3] describes another EUV radiation source based on irradiation of a continuous micro-jet of liquid nitrogen. This type of target also has the disadvantage that it does not contain enough material to have a sufficiently large number of potential EUV emitters. This is due to the relatively small diameter (about 10 xcexcm) of the liquid nitrogen jet.
Furthermore, the sources described in documents [2] and [3] are not very stable in terms of intensity. In the case described in document [2], it is difficult to irradiate each ice micro-crystal in the same manner due to a problem of synchronisation with the laser. In the case described in document [3], variations of the EUV intensity are due to instabilities of the continuous nitrogen jet.
Document [4] also describes an EUV radiation source based on irradiation of a gas jet that entrains micro particles, for example metallic micro particles. This known technique has good EUV radiation emission stability, like the technique using jets of xenon aggregates, but with a much lower particle density. The result is a significantly lower production of EUV radiation.
Furthermore, the carrier gas does not directly participate in generation of the EUV radiation. Therefore, there is a large content of superfluous gas at high frequency that has to be pumped, which causes vibrations as in the case of jets of xenon aggregates.
Documents [5] and [6] are also mentioned related to the emission of X-rays by aggregates, for example krypton, xenon or argon aggregates.
This invention divulges a technique for the generation of EUV radiation that has almost all the advantages of known techniques mentioned above, without the disadvantages of these known techniques.
The invention does this using a target consisting of dense fog of micrometric and submicrometric liquid droplets. The conversion efficiency of this target is greater than or similar to the conversion efficiency obtained with known techniques in the EUV field.
More generally, this invention relates to a process and a device for the generation of a dense fog of liquid droplets, this process and this device being useable particularly for the production of EUV radiation and is also very reliable and very simple, which is essential for industrial use.
More specifically, the purpose of this invention is a process for generation of a fog composed of liquid droplets, this process being characterised in that the liquid is pressurised at a pressure of the order of 5xc3x97105 Pa to 107 Pa and the liquid thus pressurised is injected into a nozzle that has a diameter varying between 20 xcexcm and 1 mm, this nozzle opening out into an area in which the pressure is less than or equal to 10xe2x88x922 Pa, and a dense fog of liquid droplets is thus generated at the exit from the nozzle, the sizes of the droplets being of the order of 10 xcexcm to 30 xcexcm, this dense fog being located approximately along the centre line of the nozzle. The average fog density is greater than or equal to 1020 molecules/cm3.
According to the preferred embodiment of the process according to the invention, the nozzle is heated. Generation of the dense fog absorbs calories that usually have to be compensated by the input of heat. This input of calories is lower if the liquid being used is cryogenic (liquefied gas).
Furthermore, it is advantageous to input a quantity of heat at the nozzle greater than the heat absorbed by generation of the dense fog, to improve the uniformity of the liquid droplets.
A laser beam can also be focused on the dense fog thus obtained, this laser beam being capable of interacting with this dense fog to generate light in the extreme ultraviolet range.
Advantageously, the laser beam is focused on the dense fog at a distance from the nozzle of the order of 1 mm to 10 mm.
Nevertheless, a larger distance up to several centimetres could be used without departing from the scope of the invention.
Light generated in the extreme ultraviolet range can be used for insolation of a substrate on which a photoresist layer is deposited.
Another purpose of this invention is a device for generation of a fog composed of liquid droplets, this device being characterized in that it comprises:
a reservoir that will contain the liquid,
means of pressurisation of the liquid contained in the reservoir, by subjecting it to a pressure of the order of 5xc3x97105 Pa to 107 Pa,
a nozzle, the diameter of which is between 20 xcexcm and 1 mm and that is connected to the reservoir,
a vacuum chamber containing the nozzle, and
pumping means to set up a pressure equal to or less than about 10xe2x88x922 Pa in this vacuum chamber,
to thus generate a dense fog of liquid droplets in the vacuum chamber at the exit from the nozzle, with sizes of the order of 10 xcexcm to 30 xcexcm, this dense fog being located approximately on the centre line of the nozzle.
According to one particular embodiment of the device according to the invention, this device also comprises means of heating the nozzle.
For example, the pressurisation means comprise means of injecting a pressurised gas in the reservoir.
According to one particular embodiment of the invention, the nozzle is provided with pulsing means to produce the dense fog in pulsed form.
In particular, the device according to the invention can be used with water as the liquid.
This invention also relates to a light source in the extreme ultraviolet range, this source comprising:
the fog generation device according to the invention,
means of generating a laser beam that can interact with the dense fog generated using the device, and
means of focusing the laser beam on this dense fog.
This invention also relates to lithography apparatus for semi conducting substrates, this apparatus comprising:
means of support for a semi conducting substrate on which a photoresist layer is deposited that will be insolated according to a determined pattern,
a mask comprising the determined pattern in enlarged form,
a light source in the extreme ultraviolet range,
optical means for transmission of light to the mask, the mask supplying an image of the pattern in enlarged form, and
optical means for reducing this image and projecting the reduced image on the photoresist layer,
and the source in this apparatus is conform with the light source according to the invention.